1. Field of the Invention
The present invention relates to an active matrix organic light emitting display (AM-OLED) and, more particularly, to an AM-OLED with a dielectric layer under a transparent electrode, improving the surface of the transparent electrode.
2. Description of the Related Art
In accordance with driving methods, an organic light emitting display (OLED) is an active matrix type or a positive matrix type. The active matrix OLED (AM-OLED) is driven by electric currents, in which each of the matrix-array pixel areas has at least one thin film transistor (TFT), serving as a switch, to modulate the driving current based on the variation of capacitor storage potential so as to control the brightness and gray level of the pixel areas.
Concerned with the luminescent principle of the AM-OLED, an electric current is applied to a specific organic lamination to convert electricity to luminescence. The AM-OLED has panel luminescence with thin and lightweight characteristics, spontaneous luminescence with high luminance efficiency and low driving voltage, and advantages of increased view angle, high contrast, high-response speed, full color and flexibility.
Indium tin oxide (ITO) has been widely used as anode electrode material for AM-OLED application because of its transparency, good conductivity, and high work function. Moreover, it is found that the luminescent characteristics of AM-OLED depend strongly on the surface roughness of the anode electrode. The surface roughness of ITO film must be smooth enough to avoid large leakage current or point discharge causing pixel defects.
The average roughness of ITO film formed by sputter deposition is less than 1 nm. Instead of being formed on a smooth glass plate in PM-OLED process, the ITO film is formed on an under-layer in AM-OLED process, wherein the surface roughness of ITO film depends strongly on different under-layers.
Generally, a transparent and insulating organic material is used as the under-layer in conventional AM-OLED process. However, the surface roughness of ITO film on the organic materials is 3˜4 times larger than that on a smooth glass plate. The average roughness (Ra) of ITO film on the organic materials is about 3˜4nm and such rough surface may result in large leakage current and cause point discharge, such that the luminance efficiency and lifetime of the device are adversely affected.